Driver assistance systems gather information about an area proximate to a vehicle to provide better awareness to an operator of the vehicle regarding, for example, other vehicles and objects in the travel path of the vehicle. This information can be sensed with numerous technologies. The development of affordable and reliable digital vision technology allows for providing vision based object detection systems. Monocular vision systems using a single camera or other suitable imaging device can detect objects, but have difficulty reliably and accurately determining the range to a detected object. Stereoscopic vision systems add a second camera so the system can more reliably determine the range to an object. Such stereoscopic vision systems determine range by detecting the object from the different perspectives of each camera, and then use triangulation based on a separation distance of the cameras and a position disparity of the object in the images from each camera. For example, an object that is closer to a stereo vision system will have a greater image disparity of simultaneously captured images from the two cameras when compared to that same object at a greater distance.
Stereo vision ranging performance can be improved by reducing the field of view or increasing the lens focal length of the cameras used. This has the effect of increasing the image size of objects that remain within the narrower field of view, while eliminating or truncating objects that intersect with or fall outside of that field of view. However, this option is not considered desirable for vehicle systems where a wide field of view is preferred to observe objects of interest in the distance and one that may not be directly in front of the car, such as roadside objects on a curve, or other vehicles approaching an intersection from a crossing street.
Stereo ranging performance can be also increased by increasing the resolution of the imager used in the cameras. However, increased resolution usually results in higher overall system cost and a possible reduction in overall system sensitivity, for example, reduced performance during periods of marginal lighting such as during dusk or dawn.
Stereo ranging performance can be improved by increasing the separation distance between the cameras. Maintaining proper alignment of the separate cameras is critical to the performance of the system. However, proper alignment typically requires that both cameras be mounted on a structure that is sufficiently rigid to maintain alignment of the cameras over the operating life of the system. Increasing the separation distance naturally increases the size, mass and cost of the mounting structure.